1. Field of the Invention
The present invention relates to a repurification method and apparatus for water from a public supply, particularly, but not exclusively for repurifying water (e.g. city water) from a public waterworks which has already been purified once for drinking.
2. Technology Review
In general, water purifying treatments performed in public waterworks, despite differences of treating methods and the scale of purifying equipment used, usually include a final disinfection process with chlorine. According to one water quality standard, for example, water from a tap should contain at least 0.2 ppm. of free residual chlorine or at least 1.5 ppm. of combined residual chlorine.
Though these residual chlorines maintain safety sanitarily, they can cause an unpleasant odor when drunk so it is preferable to remove them before water is used as drinking water. Further, the removal of organic materials is also required, because they provide conditions for the growth of bacteria. Thus, many kinds of water repurification equipment for removing residual chlorines and, in some cases organic materials have been developed and used.
Known types of water repurification equipment vary somewhat in operation, but normally involve the use of activated carbon having a strong ability to remove, by adsorption, the residual chlorine and organic materials by passing the water rapidly through activated carbon layers. These are satisfactory when first used, but there are many problems as listed below.
First, there is a problem with the increase in bacteria levels. The increase of bacteria has become the most important problem in the known water repurification methods.
Typically, water from the main supply has few live bacteria, but once the water has been treated by a known water repurification method, the level of bacteria is found to be 10 times to 1,000 times the permitted limit set by typical water-quality-standards.
The reason why so many bacteria are detected in the filtered water is because of the time at which known filtration processes are carried out.
A typical water repurifier filters by the following method; the inflow valve (or the service connection tap, if the repurifier is directly connected to the main supply) is opened and water passes rapidly through the layers of the activated carbon in the water purifier. After filtered water is collected in a container, the inflow valve is closed. If necessary, water can then be filtered again at any time.
In this type of filtration method, when the inflow valve is closed, water is left in the layers of the activated carbon, and loses its disinfection ability because all its residual chlorine is adsorbed by the activated carbon. In addition, on the surface of the activated carbon the dissolved oxygen volume will be increased, because air comes in through the outlet. Filtered-out organic materials are also present in the layer of activated carbon.
Consequently, in such a water repurifier, the basic conditions in which bacteria can live and multiply, namely moisture, organic material and oxygen, are present. Therefore, bacteria multiply quickly in the water repurifier and when the inflow valve is opened again to filter more water, these bacteria mix with the filtered water and flow out.
As a result, the water drunk by a consumer has a concentration of bacteria 10 times to 1,000 times greater than in the original supply.
To solve these problems, a method has been developed in which the activated carbon is coated with silver which has a disinfection ability and controls the increase of bacteria in the water trapped in the activated carbon layers. However, this method merely reduces the increase of bacteria somewhat, and the problem of exceeding the permitted limit is largely the same as in the traditional methods. Moreover, in view of the fact that silver has an injurious effect on the human body, it is questionable whether the use of silver in this way is desirable.
A second problem with typical activated carbon filters is that there is no durability of the water repurification effect. The activated carbon has a strong adsorption ability in the beginning, but as the filtered volume of water e.g. city water increases, the adsorption ability reduces very rapidly and the filtration effect decreases greatly. The reason is that the fine material and the organic material in the water become adhered and retained on the surface of the activated carbon, so that the area of the activated carbon surface which can directly act on the water is reduced and its adsorption ability is weakened.
Thus, the ability of the filter to remove residual chlorine and the organic material is comparatively good to start with, but after about one month of use, this ability is very quickly reduced, and it becomes impossible to remove turbid materials.
A third problem with carbon filters is that, the water is not filtered equally. The water is filtered to varying degrees of effectiveness, because of when the filtration method is performed and because the filtration rate may become very rapid.
A fourth problem is that washing and disinfection of the filter material is impossible. In the traditional filtration methods, washing and disinfection of the filter material has not been considered at all.
Accordingly, there has been the above problem that consumers drink filtered water contaminated with bacteria.
A fifth problem is that the change period of the filter material or of all parts of the water repurification equipment is very short.
As mentioned above, because the traditional water repurifier has no durability of filtration effect, bacteria increase, washing and disinfection is impossible, and the use period of the filter material or of the whole water repurifier is very short, so that it must be changed often.